The present invention relates to an indicator or gauge for measuring and checking the internal diameter of a gun barrel.
Shotgun barrels are provided in various sizes. The sizes are generally indicated by gauge numbers. As the gauge number increases, the internal bore diameter of the barrel decreases. For example, a standard 10-gauge shotgun has an internal bore diameter of 0.775 inches, and a standard 12-gauge shotgun has an internal diameter bore of 0.729 inches. While the gauge numbers are supposed to correspond to standard internal diameters, the actual internal diameter may be less or more than the "standard" diameter. For example, the actual internal diameter of a 12-gauge bore produced by different manufacturers may vary from as little as 0.722 inches to as large as 0.747 inches.
Shotgun barrels at the muzzle end are also "choked" to control the shot pattern. The muzzle end of the barrel is constricted by the various choke designs in order to provide a desired pattern. A full choke is supposed to deliver 70-80% of the shot to a 30-inch circle at 40 yards. A modified, or half choke, is supposed to deliver approximately 55-65% of the shot to the 30-inch circle at 40 yards. An improved cylinder choke is supposed to deliver between 45-55% of the shot to the 30-inch circle at 40 yards. For a standard gauge barrel, a specific internal diameter at the constriction is supposed to provide the desired "choked" pattern. The "standard" constrictions are not always used by the manufacturer. For example, a barrel marked "full" choke, may have a constricted diameter which corresponds to or approximates a modified choke constriction.
Also, various forms of variable choke devices are presently available. Since the actual internal diameter of the various gauge shotgun barrels produced by different manufacturers may vary from the standards, different results can be achieved when the same variable choke device is used on different shotguns. When the choke device is used on a particular gun at one setting, a different degree or amount of constriction is obtained if it is used at the same setting on a different barrel of the same marked gauge.
It is, therefore, desirable for the shotgun user to be able to check or measure the internal diameter of the barrel at various depths and also to be able to check the degree of constriction or choke at the muzzle end of the barrel. The actual internal diameter of the barrels can also provide an indication to the user of the degree of useful life or the amount of wear which has been experienced through use. As shotguns are used, internal barrel wear is necessarily experienced. The degree of constriction at the muzzle end or "choke" may decrease also during use.
Heretofore, various devices have been proposed for permitting the gunsmith and/or user to check and measure the internal diameter of the barrel. One such instrument includes an elongated, cylindrical tube or body which supports a gauge head having a plurality of resilient arms extending therefrom. A tapered, generally conically shaped rod is slidably received within the elongated tube. As the rod is shifted in the tube, the resilient arms are flexed into radial engagement with the bore of the gun barrel. The extent of axial translation of the rod is directly proportional to the internal diameter of the barrel.
Other gauges which have heretofore been proposed for measuring the internal diameter of a gun barrel also include a relatively simple X-shaped scissors mechanism which may be inserted into the gun barrel. Once inserted, the scissors mechanism is expanded about its pivot point. After expansion, the mechanism is withdrawn and the degree of expansion measured by a standard micrometer. This device may be difficult to use when measuring the barrel from the muzzle end due to the choke constriction. Another device is a relatively complex, cylindrically-shaped unit which has a conically-shaped end for biasing retained ball bearings outwardly against the internal diameter or inner surface of the barrel. A graduated gauge at the outer end of the structure is provided for directly reading the internal diameter.
Various forms of general purpose gauging or indicator tools have been proposed for indicating internal diameter. One such gauge or tool is disclosed in U.S. Pat. No. 2,642,672, entitled Hole Gauge and issued on June 23, 1953, to Lewis et al. This tool includes an elongated, conical plug attached to one end of a rod which is slidably disposed in a generally cylindrical body. A pluraity of balls are held in engagement with the conical plug by resilient arms which in turn are secured to the body. As the rod is shifted axially, the balls ride on the conical plug and are shifted radially outwardly. This gauge or tool is specifically designed for measuring the internal diameter of holes formed in the roof of a mine to receive expansion bolts. Other examples of general use internal diameter gauges may be found in U.S. Pat. No. 2,663,942, entitled Instrument for Measuring Inside Dimensions and issued on Dec. 29, 1953, to Rudolph and U.S. Pat. No. 3,589,017, entitled Bore Gauge and issued on June 29, 1971, to Kyle.
A need exists for a relatively inexpensive device which is as accurate as the complex cylindrical devices of the prior art yet which requires no additional micrometer or the like to make the measurement. Such a device would preferably possess the advantages of increased ease of manufacture, increased ease of assembly and ease of use when compared to the prior art structures. Such a device would permit a greater number of shotgun users to accurately check the internal diameter of a gun barrel and to accurately determine the degree of choke provided by the manufacturer.